Symmetry and twins in the monophosphate tungsten bronze series (PO2)(4)(WO3)(2m) (2 <= m <= 14)

Monophosphate tungsten bronze with pentagonal tunnels (PO2)(4)(WO3)(2m) are low-dimensional materials with charge density wave (CDW)-type electron ins tabilities. Two forms of the structure can thus be expected for all the mem bers of the series: a low-temperature form (LT) corresponding to the CDW st ate and a high-temperature form (HT) corresponding to a normal metallic sta te. The HT form is described here for m = 9 and compared with that of the m = 5 and m = 7 counterparts. It is shown that a systematic twin phenomenon must be taken into account for HT members because of two possible configura tions of the tilting mode of WO6 octahedra. The structure is also compared with that of m = 10, which exhibits the modulated CDW-LT form at room tempe rature. Owing to two possible polarization directions of the segments built of m WO6 octahedra, a twin phenomenon is also encountered in the LT forms. A review of all the structures known at present (m = 2, 4, 5, 6, 7, 8, 9, 10, 12) leads us to propose a structural law based on the building mode of WO6 octahedra in WO3-type slabs to explain the symmetry changes observed be tween even and odd members of the series.